Abstract: Polymerization catalyst systems which are prepared by admixing an oxide of metal of Group IIA, IIB, IIIA or IVA with a halide of a metal selected from Groups IVB, VB or VIB to chemically fix the metal halide thereon, and thereafter adding an organo Group IA, IIA or IIIA metal alkyl, metal alkyl halide or metal hydride and a Lewis Base modifier to the admixture are improved by prehaliding the oxide with a halogenating agent comprising a halide of aluminum, silicon or phosphorus or post-haliding the mixture of the metal oxide and metal halide with an additional amount of a Group IVB metal halide so that the atomic ratio of halide to Group IVB, VB or VIB metal is greater than 10:1 and preferably greater than 20:1.